Steric Effects of Meta Substituents in Substituted Tetraphenylporphyrin Complexes of Ruthenium, Indium, Titanium, and Gallium

The ruthenium carbonyl 4-tert-butylpyridine, indium chloride, titanyl, and gallium chloride complexes of tetrakis(3,4,5-trimethoxyphenyl)porphyrin and tetrakis(3,5-dimethoxyphenyl)porphyrinand the ruthenium carbonyl 4-tert-butylpyridine, titanyl, and gallium chloride complexes of tetrakis(3,5-di-tert-butyl-4-hydroxyphenyl)porphyrin and tetrakis(3,5-di-tert-butyl-4-methoxyphenyl)porphyrin have been prepared. For each metal ion, the electronic spectra were used to monitor the electronic effects of the porphyrin substituents. Rates of phenyl ring rotation were obtained from the variable-temperature NMR spectra of the ortho protons and of the meta substituents. The rate of exchange between free 4-tert-butylpyridine and 4-tert-butylpyridine coordinated to the ruthenium complexes was obtained from the NMR spectra of the tert-butyl groups. The rates of phenyl ring rotation inthe meta-substituted complexes are 7-10 times slower than those expected on the basis of the previously observed electronic effects on phenyl ring rotation. Rates of 4-tert-butylpyridine exchange ar 2-3 times faster than those expected on the basis of electronic effects. The reduced rate of phenylring rotation and the enhanced rate of 4-tert-butylpyridine exchange are attributed to a steric effect of the meta substituents.